PT DOC

I have a 08 A6 Z51 with about 10k miles on it. I bought the car about 8 months ago. It has Corsa sport exhaust and Honeybee CAI (done by original owner). Car has been perfect until a couple of weeks ago. It began to make a "grinding" / "vibrating" like noise from what sounds like the rear. After searching, I figured it sounded like the dreaded differential TSB. So, I drained the diff and replaced with AMSOIL and additive, did some figure 8s and the sound is still there.

It sounds like it could be the exhaust but the noise is not consistent and although it is reproducible, even that is not consistent. It usually occurs after the car has warmed up (about 10-15 minutes). The noise is most noticeable at about 1500 - 2000 rpms and only when in gear, usually while accelerating. It occasionally occurs when stopped, in drive OR reverse.

I have been under the car several times and everything appears tight - I retorqued the exhaust clamps and the sway bar connectors.

It does NOT occur consistently with turning (as most people report with the Diff fluid problem) and it only occurs as the car gets warmer. I know some people had to change the diff fluid twice but I'm not sure the noise is consistent with the diff problem.

I did read some other posts with similar noises related to the "torque tunnel" and possible "baffle" issues or other exhaust problems. But, I am unsure if these could be the culprit.

Anybody that replaced the diff fluid have similar symptoms?

Any ideas??

Thanks.

Larry/car

You went through all the trouble, why didn't you put the right fluid in it? I did mine last Saturday, the noise went away before I left the garage. I did spin the wheels manually with the car jacked up and it sat for 36 hours before being driven. I didn't need figure eights and didn't perform them. I am Happy with the results. Now you have some foreign fluid in it and maybe you will have to flush the system.

PT DOC

Larry/Car

Glad your problem is better. But what where your symptoms? Did the noise only occur with turns and at slow speeds while the car was cold (like many others have described) or where they similar to mine.

Also, many others have used the AMSOIL with great outcomes. But if it's necessary to do it again I will use the GM recommended product. It only takes a half hour, just don't want to change fluid again if that's not the problem.

cclive

The rear gear fluid problem only presents itself when turning sharply at slow speeds...mine only did it when it was cold coming out of the garage. Because of that, it took a second trip to the dealer to get it fixed.

PT DOC

Well, after more researching, it's looking more and more like a torque tube problem. Anyone ever have work done on the torque tube bearings (or any other part inside the tube) that sounds similar?
If so, what kind of damage ($$) am I looking at? From what I see, it looks like "cheap" parts and lots of labor.

C66 Racing

If your noise is persistent when driving straight - it is not the diff fluid as the diff clutch plates have no relative motion against each other in this case. Numerous other forum members have used the AMSOIL diff fluid and additive to cure the C6 diff chatter. Suspect you have something else that you are hearing.

Dave S

There have also been reported problems with the rear axle nuts backing off, resulting in noise from the rear end. If memory serves me...solution is to replace the axle nuts (not re-torque the original nuts) using Locktite 272 & torque to 140 lbs. The replacement nuts are cheap (p/n 10257766) & might be worthwhile to pursue before jumping into the torque tube bearings. Gene Culley (GM Parts House) can supply the parts & you can do a Forum search for "axle nut problem" or something similar. Note that the Locktite needs to cure 24 hours after application. Hope this helps....

PT DOC

Thanks for the replys. I will look into the suggestions. Really hope it's not the torque tube.
Thanks again.

bearphoto

I concur with DaveS, I'd check or replace those axle nuts before doing anything else. Dont worry about the Amsoil in the rear diff, its one of the best fluids out there, that is of course if you used the correct diff fluid and additive.

While i have heard of some torque tube issues my belief is that type of problem is pretty rare. You hear alot more issues about the axle nuts then the torque tube.

PT DOC

Ordered the axle nuts from Gene. I'll let everyone know. Fingers crossed.
Thanks again

RJRSW

The fluid change fixed my jerking, chattering and other noises coming from the differential but I used the recommended fluid and make sure you do not add the friction modifier additive.

My dealer also retorqued the axle nuts that also can contribute to the problems you are having.

Quote from the #07-04-20-002A - (01/09/2008 TSB:

"Subject: Rear Axle Clunk and/or Chatter Type Noise on Turns (Drain/Refill Rear Differential Fluid) #07-04-20-002A - (01/09/2008)

Models: 2004-2008 Cadillac XLR (Including V-Series and Export)

2005-2008 Chevrolet Corvette (Including Z06 and Export)

--------------------------------------------------------------------------------

This bulletin is being revised to change the fluid recommendation and to remove the requirement of adding friction modifier. Please discard Corporate Bulletin Number 07-04-20-002 (Section 04 -- Driveline/Axle).

Parts Information
Part Number
Description

88862624 (US-1 Quart)"

It takes about 2 quarts of the above fluid to refill.

cclive

Did you check to see if your axle nuts are loose?

RJRSW

Make sure you get the new updated torque spec because it was changed by GM according to the service writer I always use at the dealership. They did not need to replace the nuts but just torque them properly.

They do a very large Corvette service business at the Chevy dealer that I use and even have a special protected area for the corvettes to be parked in while waiting to be serviced and very experienced corvette certified techs that work on them.

AYello1

What IS the "new updated torque spec"?

Gearhead Jim

Better check on that. The bulletin has changed somewhat over the years, but last I heard they were saying not to retorque the nuts; get the new style nuts, install with red Loctite, wait 24 hrs. There have also been some changes to the torque specified when installing the new nuts.

Raazor

RJRSW

Mine was done at the dealer nearly a year ago and maybe there are some new requirements since mine was done. The dealer I use is very knowledgeable and very good on Corvette service and I wasn't aware of the bolt re-torque until I brought the car in for an oil change and they mentioned that they would do it after they changed the oil. He checked the service history on the car and found it had not been done already so they did it then. They just did it and there was no charge for it on my bill.

C66 Racing

Checking the torque on the rear axle nuts is very easy... assuming you have a torque wrench that can go high enough and the right size socket. My shop manual is at my brother-in-law's right now (and it's C5 anyways) so I won't quote numbers. Just pop off the center wheel cap, put the torque wrench on the axle nut and see what you have. If it is low, you'll easily find out.

HOXXOH

The '08 Service manual has the nut torque listed as 151# and no mention of discarding the old nut or using locktite.

John Harry

My notes from posts by Talon90 and others say that if the spindle nut moves it must be replaced with part number 12457766, using 272 Loctite and torqued to 140 ft lb. Reference the following bulletins: 07-04-95-001A: for the repair procedure, and 08-04-20-003: Half Shaft (Drive Axle) Nut Appears To Be Loose (indicating that if the nut loosens it must be replaced, and the other bulletin is referenced for repairs.)